1. Field of the Invention
The present invention relates to an active noise control apparatus for reducing an in-compartment noise with a cancellation sound, which is opposite in phase to the in-compartment noise, and more particularly to an active noise control apparatus for reducing a drumming noise (hereinafter also referred to as “road noise”), which is generated in the passenger compartment of a vehicle while the vehicle is running.
2. Description of the Related Art
Heretofore, there has been known in the art an active noise control apparatus (hereinafter also referred to as a “periodic-noise-compatible ANC”) for reducing noise (hereinafter referred to as “engine muffled sound” or “engine noise”) caused by a vibratory noise, which is produced by a vibratory noise source such as an engine or the like on a vehicle and generated periodically in synchronism with the rotation of the engine, by generating a control signal via a control unit, for canceling the engine noise based on a signal that is highly correlated to the vibratory noise produced by the vibratory noise source, and outputting a canceling sound, which is opposite in phase to the engine noise based on the control signal, into the passenger compartment of the vehicle (see Japanese Laid-Open Patent Publication No. 2004-361721).
While the vehicle is running, vibrations of the tires caused by the road are transmitted through suspensions to the vehicle body, thereby producing an aperiodic drumming noise (road noise) in the passenger compartment. The road noise constitutes a non-periodically generated low-frequency noise generated in the passenger compartment, and is produced as a resonant sound having a high sound pressure level at a certain frequency (resonant frequency), due to the resonant characteristics of the passenger compartment. Therefore, the resonant sound is defined by the road noise having a central frequency equal to a certain resonant frequency f of 40 [Hz], for example.
Japanese Laid-Open Patent Publication No. 2000-322066 discloses an active noise control apparatus (hereinafter also referred to as an “aperiodic-noise-compatible ANC”) including a plurality of microphones installed in a passenger compartment. The microphones generate canceling error signals based on differences (hereinafter also referred to as “canceling error sounds”) between the noise in the passenger compartment and a canceling sound, and output the generated canceling error signals to a control unit. The control unit generates a control signal based on the canceling error signals, and a speaker outputs a canceling sound based on the control signal into the passenger compartment. In this manner, road noise is reduced by the canceling sound according to a feedforward control process. Japanese Laid-Open Patent Publication No. 2000-322066 also reveals that microphones are used to detect noise in the passenger compartment, a control unit, which is in the form of an analog circuit, generates a control signal based on the noise, and that a speaker outputs canceling sounds based on the control signal into the passenger compartment. In this manner, road noise is reduced by canceling sounds according to a feedback control process.
Japanese Laid-Open Patent Publication No. 2001-282255 discloses that a speaker is shared by a periodic-noise-compatible ANC and/or an aperiodic-noise-compatible ANC (hereinafter also referred to simply as an “ANC”) and an audio system on a vehicle, so that the speaker can output sounds based on an output signal from the audio system, and a canceling sound based on a control signal from the ANC, into the passenger compartment of the vehicle.
Engine noise referred to above is defined as periodically generated noise within a narrow frequency band having a predetermined central frequency. A periodic-noise-compatible ANC generates a control signal having a control frequency depending on the predetermined central frequency, and the speaker outputs canceling sounds having the control frequency into the passenger compartment for effectively reducing noise in the passenger compartment.
Road noise is defined as aperiodically generated low-frequency noise having a central frequency equal to a resonant frequency of 40 [Hz], for example, which is determined from the resonant characteristics of the passenger compartment. An aperiodic-noise-compatible ANC is required to reduce resonant sounds at respective resonant frequencies.
If the aperiodic-noise-compatible ANC generates a control signal according to a feedforward control process, then the control unit needs to comprise a FIR adaptive filter and a DSP (Digital Signal Processor) for performing convolutional calculations at the respective resonant frequencies. As a result, the aperiodic-noise-compatible ANC is relatively expensive to manufacture. Furthermore, since the aperiodic-noise-compatible ANC generates a control signal at the resonant frequencies, while updating the filter coefficient of the adaptive filter from time to time, the control unit suffers from an increased computational burden in connection with generating the control signal.
If the aperiodic-noise-compatible ANC generates a control signal according to a feedback control process, then the control unit needs to comprise a combination of several analog filters for generating a control signal at the resonant frequencies. As a result, the control unit requires a large circuit scale, thereby causing the ANC including the control unit to have a large unit size. However, it is difficult for an ANC having such a large unit size to find sufficient installation space inside the vehicle. In addition, it is also difficult to combine the ANC having such a large unit size with a digital audio unit.
An aperiodic-noise-compatible ANC has been considered for generating a control signal according to a feedback control process based on a digital signal processing method, to thereby silence an aperiodic resonant sound (resonant noise).
FIG. 18 of the accompanying drawings shows an aperiodic-noise-compatible ANC 200 comprising a microphone (canceling error signal detector) 18 and a speaker (sound output device) 22, which are disposed in the passenger compartment 14 of a vehicle, and a control unit 50. The control unit 50 comprises an A/D converter (ADC) 59, a controller 202 in the form of a microcomputer and having a given transfer function H, and a D/A converter (DAC) 65. The aperiodic noise includes a resonant sound (aperiodic resonant noise), which is aperiodically generated inside the passenger compartment 14, and which has a high sound pressure level at a certain resonant frequency f due to the configuration of the passenger compartment 14.
It is assumed that, at a time t(n−1) of a sampling event (n−1), the controller 202 generates a control signal y(n−1) in the form of a digital signal for canceling out noise (aperiodic noise) in the passenger compartment 14. Then, the DAC 65 converts the control signal y(n−1) into an analog signal, and the speaker 22 outputs a canceling sound into the passenger compartment 14 for canceling out the noise, based on the analog control signal y(n−1).
The microphone 18 is located at an antinode of the acoustic mode of the passenger compartment 14. At a time t(n) of a sampling event n, the microphone 18 outputs a canceling error signal e(n) to the ADC 59, representing a difference (canceling error sound) between the canceling sound and the noise.
Specifically, at the sampling event n, a canceling sound at the position of the microphone 18 is defined as a canceling sound that has been output from the speaker 22, based on the control signal y(n−1) from the controller 202 at the preceding sampling event (n−1), and that has reached the microphone 18. If the transfer characteristics from the speaker 22 to the microphone 18 with respect to the sound at the resonant frequency f are represented by C, then the canceling sound (the signal depending thereon) at the position of the microphone 18 at the sampling event n is represented by C·y(n−1). The transfer characteristics C are divided into gain characteristics (amplitude change) G′ and a phase delay (phase characteristics) φ′. At the sampling event n, the resonant noise (the signal depending thereon) having a resonant frequency f at the position of the microphone 18 is represented by d(n).
Therefore, the canceling error signal e(n) output from the microphone 18 to the ADC 59 is expressed according to the following equation (1):e(n)=d(n)+C·y(n−1)  (1)
The ADC 59 converts the canceling error signal e(n) from an analog signal into a digital signal, and outputs the digital canceling error signal e(n) as an input signal x(n) to the controller 202. Based on the input signal x(n) {=e(n)}, the controller 202 generates a control signal y(n) {=−d(n+1)/C} depending on the canceling sound C·y(n), which is opposite in phase with a resonant noise d(n+1) at the position of the microphone 18.
According to the silencing control process carried out by the ANC 200 to silence the resonant noise, it is important to decide how to generate the control signal y(n) for the canceling sound C·y(n), which is opposite in phase with the resonant noise d(n+1) at the position of the microphone 18.
If it is assumed that the control signal n(y−1) is generated at the preceding sampling event (n−1) and the resonant noise d(n) at the position of the microphone 18 happens to be completely silenced by the canceling sound C·y(n−1) at the present sampling event n, then since the canceling error signal e(n) output from the microphone 18 is expressed by e(n)=d(n)+C·y(n−1)=0, the signal x(n) input to the controller 202 is expressed by x(n)=e(n)=0.
Since x(n)=0 regardless of the resonant noise d(n) present at the sampling event n, the controller 202 is unable to generate a control signal y(n) and the speaker 22 is unable to output a canceling sound. Therefore, the resonant noise d(n+1) at the position of the microphone 18 cannot be silenced. Alternatively, the controller 202 fails to generate a highly accurate control signal y(n), and even if the speaker 22 outputs a canceling sound, the resonant noise d(n+1) at the position of the microphone 18 cannot be silenced completely and the resonant noise d(n+1) remains unsilenced. As a result, the resonant noise d(n+1) cannot be silenced stably at the next sampling event (n+1).